1. Field of the Invention
The present invention relates to an acceleration sensor, and more particularly, it relates to an acceleration sensor which is formed by a bimorph piezoelectric element.
2. Description of the Background Art
In general, an acceleration sensor employing a bimorph piezoelectric element is known in the art. An example of such an acceleration sensor is now described with reference to FIG. 1.
An acceleration sensor 1 comprises a sensor body 2 which is formed by a bimorph piezoelectric element, and an insulated case 3. The acceleration sensor 1 is generally mounted on a mounting substrate 4, as shown in FIG. 1.
The sensor body 2 is formed by a combination of first and second piezoelectric ceramic plates 5 and 6. First and second signal electrodes 7 and 8 are formed on outer major surfaces of the first and second piezoelectric ceramic plates 5 and 6 respectively. The first and second signal electrodes 7 and 8 are opposed to each other through the piezoelectric ceramic plates 5 and 6 at a central portion of the sensor body 2. Further, an intermediate electrode 9 is formed between the first and second piezoelectric ceramic plates 5 and 6. Namely, the first and second piezoelectric ceramic plates 5 and 6 are pasted to each other through the intermediate electrode 9. The intermediate electrode 9 is formed along the portion where the first and second signal electrodes 7 and 8 are opposed to each other.
The first and second piezoelectric ceramic plates 5 and 6 are polarized as shown by broken arrows in FIG. 1. In other words, the first and second piezoelectric ceramic plates 5 and 6 are uniformly polarized along thicknesses thereof, in directions which are opposite to each other.
The first and second signal electrodes 7 and 8 are drawn out on first and second ends along the longitudinal direction of the sensor body 2, respectively.
The insulated case 3 has first and second holding members 10 and 11 and flat plate type case substrates 12 and 13. The first and second holding members 10 and 11 are fixed to outer major surfaces of the sensor body 2 in portions close to both ends thereof, respectively. The first and second holding members 10 and 11 have cavities 10a and 11a, respectively. These cavities 10a and 11a are adapted to define spaces for allowing displacement of the sensor body 2. Thus, the sensor body 2 is supported by the first and second holding members 10 and 11 in the form of a center beam.
Still another cavity 12a is formed in an upper surface of the case substrate 12, in order to define a space for allowing displacement of the sensor body 2. A further cavity is also formed in a lower surface of the case substrate 13 which is shown in phantom lines in FIG. 1, to allow displacement of the sensor body 2.
The first and second holding members 10 and 11 are fixed to both sides of the sensor body 2 and the case substrates 12 and 13 are bonded to upper and lower portions thereof as described above, so that the sensor body 2 is stored in the insulated case 3. Terminal electrodes (not shown in FIG. 1) are formed on both end surfaces of the insulated case 3, to be connected with the first and second signal electrodes 7 and 8 respectively.
FIG. 2 is a perspective view showing another exemplary conventional acceleration sensor 21. The acceleration sensor 21 has a sensor body 22 which is similar in structure to the sensor body 2 of the acceleration sensor 1 shown in FIG. 1. Since the sensor body 22 itself is similar in structure to the sensor body 2, portions identical to those in FIG. 1 are denoted by similar reference numerals.
In the acceleration sensor 21, the sensor body 22 is so arranged that major surfaces of first and second piezoelectric ceramic plates 5 and 6 are in a direction which is parallel to a major surface 4a of a mounting substrate 4. First and second holding members 23 and 24 are arranged on upper and lower portions of the sensor body 22 respectively. The first and second holding members 23 and 24 are similar in structure to the first and second holding members 10 and 11 shown in FIG. 1. The acceleration sensor 21 is different from the acceleration sensor 1 in that the sensor body 22 is arranged in the aforementioned direction while the first and second holding members 23 and 24 and case substrates 25 and 26 are arranged in response to the direction of the sensor body 22, and the respective elements are similar to those of the acceleration sensor 1.
In the acceleration sensors 1 and 21 shown in FIGS. 1 and 2, the sensor bodies 2 and 22 exhibit excellent sensitivity with respect to acceleration acting along thicknesses thereof. However, the sensor bodies 2 and 22 have no sensitivity with respect to acceleration acting along widths thereof. In other words, there are specific insensitive directions Q along which no acceleration is detectable.
When a bottom surface 3a of the insulated case 3 is fixed onto the major surface 4a of the mounting substrate 4 for mounting the acceleration sensor 1 on the mounting substrate 4, therefore, the sensor body 2 exhibits maximum sensitivity with respect to acceleration acting along a direction X which is parallel to the major surface 4a, while the same has no sensitivity with respect to acceleration acting along a direction Z which is perpendicular to the major surface 4a.
When the acceleration sensor 21 is similarly mounted on a major surface 4a of a mounting substrate 4 as shown in FIG. 2, further, the sensor body 22 exhibits maximum sensitivity with respect to acceleration acting along a direction Z which is perpendicular to the major surface 4a, while the same has no sensitivity with respect to acceleration acting along a direction X which is parallel to the major surface 4a.